Jailbreaking team iPhoneIslam has managed to enable FaceTime on a jailbroken iPhone 3GS... sort of. iPhoneIslam's app, "FaceIt-3GS," is currently available in the Cydia store, and allows users to enable FaceTime in the iPhone 3GS's Settings app.
However, the functionality is quite limited. The most obvious stumbling block is hardware-related -- without a front-facing camera, the FaceTime experience will be quite limited on the iPhone 3GS no matter how creative jailbreakers get. iPhoneIslam's FaceIt-3GS app doesn't enable FaceTime via the 3GS's rear-facing camera either, though, so the only added functionality this hack appears to give 3GS owners is the ability to accept FaceTime calls from an iPhone 4, newest-generation iPod touch, or Mac, and view the sender's video stream.
Not being able to use the rear camera for sending video is disappointing, but if your iPhone 3GS is already jailbroken, you might want to give this a try anyway. Only being able to view someone else's FaceTime video is limiting, but it's better than having no FaceTime on the 3GS at all. You can see a video of the hack at work above -- but make sure to turn your volume down before you get to the end of the video.